This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We propose multi-edge x-ray absorption studies on a series of Cu and Fe containing biomimetic complexes to gain insights into the non-innocent nature of metal-ligand bonding interactions. While these complexes contain abiotic ligands with phosphine, phosphido, and amido groups in addition to biotic thiol and thioether groups, they provide a systematic and tunable way to probe electronic and geometric structure contributions to biological redox processes in electron-transfer CuA-sites and small-molecule activation centers such as hydrogenases and nitrogenases. The proposed XANES analyses of rising-edge features at metal L-edge and ligand K-edges will provide direct experimental information about the ligand-based redox chemistry and the extent of metal-to-ligand back-donation. The EXAFS analysis at the metal K-edge will be used to characterize the sample integrity with reference to small-molecule crystallography. The interpretation of the spectroscopic data will be aided by electronic structure calculations. Furthermore, the computational level of theory (basis set and functional) will be calibrated to metal and ligand characters that will be experimentally determined from pre-edge and rising-edge features.